The present invention relates to ultrasonic inspection systems and more particularly to such a system which may be used to map the soft tissues of a patient's body and for other medical diagnostic purposes and for determination of the internal structure of trees.
Present methods of gaining information on the internal structure of trees are largely based on experience, experimental borings, or hammer soundings. The bid made by the lumberman to the owner of a stand of timber depends largely on his estimate of the relative amounts of heartwood and softwood in certain species in that stand of trees. The amount the lumberman is paid for his cut timber depends on the amounts of heartwood and softwood in some species such as maple or white birch which he delivers at the mill. A method of measuring the relative amounts of heartwood and softwood in the standing tree, as well as defects inside the standing tree, could thus be of great utility to the lumberman.
Lumber mills receive logs with essentially unknown internal characteristics and then attempt to cut the maximum usable lumber from each log. An instrument capable of detecting internal tree structure which could act as a data base for computer controlled cutting could maximize the usable lumber obtained from a given log. Some experimentation with presently available echo sounding equipment has been attempted for this purpose, but without much success, for the same reasons which limit the analysis of animal tissues, discussed below.
At the present time ultrasonic inspection systems generally comprise an ultrasonic transducer, such as piezoelectric element which produces bursts of sound waves at a high frequency. The sound waves may be directed as a beam towards the material to be inspected. The material is submerged in a liquid medium along with the propagating transducer and receiving transducer, or both transducers are coupled to the material. The receiving transducer is generally arranged to receive an echo (reflected wave) from the propagated ultrasonic waves. The echoes are analyzed, by amplitude and delay time after the transmitted burst, to determine the nature and depth of the spot being inspected. Such inspection systems, particularly as applied to medical diagnosis, have distinct drawbacks. The information obtained about the nature of the point from which the echo is returned is difficult to analyze because the orientation of the reflecting surface is unknown. In addition, as reflections are returned only from surfaces, large portions of space are totally unrepresented, i.e., those systems do not provide a value for each point of tissue, particularly soft tissue, in space.
There is need in medicine for measurements of soft tissue characteristics in terms of some parameter which is strongly biologically related, and also a great need for techniques which can measure the blood supply in various tissues.
Present methods of gaining information on soft tissues by means of ionizing radiation are largely related to the types of atomic neuclei present in the tissue, rather than any other characteristic of the tissue such as the degree of cell architecture, or the protein content of the tissue. In the case of X-rays the absorption coefficient is heavily dependent on the atomic number of the neuclei present, and thus the bony structure shows up clearly because of its calcium content. But soft tissues are all much alike from the point of view of the absorption of the X-rays. To gain information on soft tissue characteristics, it is necessary to introduce foreign neuclei (elements) of high atomic number into the patient and follow the distribution patterns of these new chemical elements as a way of tracing flows of materials in the body.
In radioactive tracer techniques foreign materials are introduced into the patient, and their radiation followed. Again the information is limited to finding the preferential positions to which these foreign neuclei flow. The properties of the tissue itself and the extent of any neoplasm (cancer) is a matter of conjecture based upon the distortion of the "normal" flow pattern. In addition, the resolution of that method is quite poor, being one centimeter at best.
Various sonic and ultrasonic techniques have been proposed for medical diagnoses, and a number tried. In the low frequency range, auscultation has been a part of the medical arsenal for many years. In the ultrasonic range, transmission methods have been tried with limited success. One difficulty is that the measurement determines the total absorption of the sound beam in going through the entire body, not the absorption at a point. Under these conditions, the bones produce such a large portion of the absorption that the effects of the soft tissue are essentially negligible and, when they are visible at all, they are spatially localized only to the extent that they are somewhere along the rather broad propagation path of the sound beam.
Echo techniques using ultrasonics (referred to as echography in the medical literature) are not the common technique to which reference is made at present when "diagnostic ultrasound" is discussed. These techniques all depend on an incident ultrasonic burst being reflected from a point in the tissue. Echoes or reflected signals occur because of changes in the acoustic impedance of the tissue. Acoustic impedance is the product of the sound velocity and the mass density of the tissue. The more abrupt this change is, the more energy will be reflected as an echo. The magnitude of the echo signals is thus large only for surfaces between tissues and is quite small for soft tissue, as both velocity and mass density are nearly constant in soft tissue. The ultrasonic echogram is thus capable of detecting interfaces, but is not presently capable of measuring the characteristics of every point in the tissue.